WBR0907
| Author | [[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Serge Korjian)]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Biochemistry |
| Sub Category | SubCategory::General Principles |
| Prompt | [[Prompt::A pregnant woman known to have classic galactokinase deficiency is worried she will not be able to breastfeed her baby because she has been on a strict galactose free diet since her birth. The physician comforts her and tells her that she will be having normal breast milk for lactation. The ability of the mammary glands to synthesize milk in the absence of dietary galactose in this patient is due to the activity of which of the following enzymes?]] |
| Answer A | AnswerA::Aldose reductase |
| Answer A Explanation | AnswerAExp::Aldose reductase catalyzes the conversion of galactose into galacticol and glucose to sorbitol. |
| Answer B | AnswerB::Galactokinase |
| Answer B Explanation | AnswerBExp::Galactokinase catalyzes the conversion of galactose to galactose-1-phosphate. The enzyme is deficient in this patient, but it is not directly involved in the synthesis of lactose. |
| Answer C | AnswerC::Galactose-1-phosphate uridyltransferase |
| Answer C Explanation | AnswerCExp::Galactose-1-phosphate uridyltransferase (GALT) catalyzes the reversible conversion of galactose-1-phosphate into UDP-galactose. This enzyme is deficient in patients with classic galactosemia. |
| Answer D | AnswerD::UDP-galactose-4-epimerase |
| Answer D Explanation | [[AnswerDExp::UDP-galactose-4-epimerase (GALE), also known as UDP-glucose-4-epimerase, catalyzes the reversible conversion of UDP-galactose to UDP-glucose. In the absence of an exogenous source of galactose, GALE catalyzes the formation of UDP-galactose necessary for the synthesis of lactose.]] |
| Answer E | AnswerE::UDP-glucose-pyrophosphorylase |
| Answer E Explanation | AnswerEExp::UDP-glucose-pyrophosphorylase catalyzes the conversion of UDP-glucose to glucose-1-phosphate. The enzyme is not directly involved in the synthesis of lactose. |
| Right Answer | RightAnswer::D |
| Explanation | [[Explanation::The patient suffers from galactokinase deficiency, a genetic disease of galactose metabolism. Patients with galactokinase deficiency are required to have a galactose free diet to avoid the accumulation of galacticol in tissues most commonly manifested as early onset cataracts. Even in the absence of dietary galactose, the endogenous synthesis of galactose can occur. Galactose is produced as UDP-galactose by the activity of the epimerase enzyme on UDP-glucose. The synthesis of the disaccharide lactose (the primary milk sugar) in the mammary glands occurs via lactose synthase with UDP-galactose and UDP-glucose as substrates.
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| Approved | Approved::Yes |
| Keyword | WBRKeyword::Galactose, WBRKeyword::Lactose, WBRKeyword::Glucose, WBRKeyword::Galactokinase deficiency, WBRKeyword::Breastfeeding, WBRKeyword::Epimerase |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |
